1. Field of the Invention
The present invention disclosed in the specification relates to a semiconductor device with a digital signal as an input signal. For example, the present invention can be utilized in a liquid crystal display device of an active matrix type, an EL (electroluminescence) display device and so on. More accurately, the present invention can be utilized in a drive substrate of a liquid crystal display device of an active matrix type, an EL display device and the like.
2. Description of Related Art
There is provided a liquid crystal display device of an active matrix type as an example of a semiconductor device with a digital signal as an input signal.
Conventionally, a liquid crystal display device of an active matrix type with a digital signal as an input signal is provided with a constitution as shown by FIG. 1.
A signal dividing circuit 102 receives an input of a digital signal for constituting an input signal (hereinafter, input digital signal) and outputs a modified digital signal of which a pulse length is expanded over time (although the length may be expanded by any magnification, it is most general to expand it by a magnification of m) to a modified digital signal line 112. The expansion over time of the pulse length of the input digital signal by a magnification of m signifies, in other words, a reduction in the frequency of the input digital signal by a magnification of 1/m.
Although respective single ones of input digital signal line 111 and the modified digital signal line 112 are illustrated in FIG. 1, actually, there are n of the input digital signal lines and m×n of the modified digital signal lines. “n” designates a natural number and “m” designates a natural number of 2 or higher. Further, m of the modified digital signals in correspondence with m of the consecutive input digital signals in the respective input digital signal lines, are outputted to m of the modified digital signal lines separated from each other. That is, in respect of an arbitrary one of m×n of the modified digital signal lines, two of the consecutive modified digital signals correspond to two of the input digital signals which are disposed at the interval of m thereof in a certain input digital signal line. Numeral 101 shows a digital signal source.
FIG. 9 shows an example of timing charts in the case of n=2 and m=2. In reference to FIG. 9, modified digital signals of SD1 or the like are outputted at one of four modified digital signal lines and two consecutive modified digital signals P and Q respectively correspond to A and C of signals DS1 transmitted by one of input digital signal lines. Similarly, W and X in signals SD2 outputted to other modified digital signal line respectively correspond to D and F of the signals DS1. Further, s and u of signals SD3 outputted to other modified digital signal line respectively correspond to g and i of signals DS2 transmitted by other input digital signal line.
A signal line drive circuit 104 and a scanning line drive circuit 105 shown in the FIG. 1 receive modified digital signals from the modified digital signal line 112, convert them into gradation voltage signals at predetermined timings and write them to predetermined pixels.
A pixel matrix unit 106 is arranged with the respective pixels to which the gradation voltage signals are written in a shape of a lattice or in a shape of substantially a lattice (for example, delta arrangement or the like). Further, the pixel matrix portion displays a picture image of one screen by a total or a portion thereof. Numeral 103 shows a substrate having an insulating surface; for example, a glass.
Conventionally, the signal dividing circuit is provided with the following considerable significance. That is, the input digital signal is normally of several 10 MHz (in future, input digital signals of one hundred and several tens MHz may become general). However, under such a high frequency condition, function of a transistor in the drive circuit is insufficient, the operation of the circuit is not feasible or devoid of reliability. Hence, it is indispensable to reduce the frequency of the input digital signal to a degree whereby the drive circuit can be operated sufficiently and the signal dividing circuit plays a role of reducing the frequency of the input digital signal.
However, even when the function of the transistor in the drive circuit is promoted, the signal dividing circuit is not immediately dispensed with. The function of the transistor is not the only factor for enabling the operation of the drive circuit under the high frequency condition.
Firstly, there is a problem caused by resistance or capacitance. In a real liquid crystal display device, the scale of the drive circuit is large and accordingly, lines for transmitting signals necessary for operating the drive circuit from outside and power source lines are prolonged, resulting in a resistance. Further, a number of elements are connected to the respective lines and therefore, large load capacitance is added. Then, when the frequency of a signal transmitted from outside is high, there may cause a hazard in normally operating the drive circuit such that the signal becomes considerably blunted in the drive circuit, when a voltage value of the power source line is instantaneously changed by influence of certain operation in the drive circuit, a time period permitted for the recovery becomes deficient and so on.
For example, a shift register is used in the drive circuit and in inputting a clock signal of the shift register, the clock line is long and a number of clocked inverters are connected. Therefore, the shift register may not be operated normally when the bluntness of the clock signal exceeds a limit at a midway and the clock signal cannot be read at a predetermined timing.
By contrast, an area of the signal dividing circuit is normally much smaller than an area of the drive circuit and therefore, power lines or respective signal supply lines are short, connected load capacitance is also small and accordingly, even when a frequency of a signal from outside necessary for operation is high, there is no hazard in normal operation as in the drive circuit.
Hence, by reducing the frequency of the input digital signal by a magnification of 1/m by using the signal dividing circuit, the frequency of a signal necessary for operating the drive circuit can be reduced by the magnification of 1/m and occurrence of inconvenience as mentioned above which is caused in the drive circuit when the input digital signal or other signal transmitted from outside is at high frequencies. In this case, although to what degree the frequency of the input digital signal is to be reduced, needs to determine specifically in respect of individual drive circuits, it is normally sufficient to reduce it to 20 MHz or lower.
Secondary, there poses a problem of matching of timings of signals. Even in the case of transistors capable of operating under high frequency conditions, a dispersion to some degree is obliged to cause in response speeds of the individual transistors. The drive circuit is constituted by integrating a number of transistors and therefore, a shift in respect of a predetermined timing is produced which is caused by integration of the dispersion, however, the magnitude of the shift is not dependent on high or low of the frequency. Accordingly, the higher the frequency, the larger the relative influence of the shift and the higher the probability by which a total of the drive circuit does not perform normal operation is increased.
To what degree the frequency of the input digital signal is to be reduced to avoid the danger, is actually determined specifically and empirically in respect of individual drive circuits. However, the dispersion in the response speeds of the individual transistors needs to be roughly equal to or lower than 20 MHz in consideration of current fabrication steps of transistors.
Next, a description will be given of transistors used in circuits of the signal dividing circuit, the drive circuit and the pixel matrix unit.
It is not indispensable to use transistors in the circuit of the pixel matrix unit, different from the signal dividing circuit and the drive circuit. However, a screen having excellent quality in which interference of voltage information among pixels is restrained is realized by controlling voltage information written to respective pixels by using transistors, that is, by adopting an active matrix system. The transistor is required to be present on a substrate for transmitting visible light in a very small scale (much smaller than size of pixel, representatively, about 20 micrometers square) and therefore, a thin film transistor (abbreviated as TFT) is used.
Currently, there are a case in which TFTs are used in the drive circuit and a case in which ICs (Integrated Circuit) of MOSFETs (Metal Oxide Silicon Field Effect Transistor). When TFTs are used, the drive matrix unit and the drive circuit can be formed simultaneously on a substrate (which is referred to as integral formation) which contributes to reduction in production steps or expense by that amount. When ICs are used, they are used by mounting them externally on a substrate, although wirings for connecting ICs and the pixel matrix unit are needed, the function of TFT in the pixel matrix unit may be low which is an advantage. When ICs are used, there is a chip on glass (COG) system in which ICs are pasted on a substrate where the pixel matrix unit is formed.
In the signal dividing circuit, TFTs have not been used and only externally mounted ICs have been used. The reason is that the quality of a silicon film used in channels of TFTs has not been sufficient. As mentioned above, the input digital signal is normally of several tens MHz, however, under such a situation, the field effect mobility of carriers of TFT has been about 50 cm2/Vs even with a polycrystal silicon (polysilicon) film which has best quality among silicon films which have been used in channels of TFTs and therefore, in reality, TFTs cannot be driven under a high frequency condition of 10 MHz or higher.
However, when the pixel matrix unit and the drive circuit are integrally formed by TFTs, according to the method in which externally mounted ICs are used in the signal dividing circuit, extra steps and expense are needed for the externally mounting operation and an increase in production cost is unavoidable by that amount. Therefore, advantage of integral formation of a reduction in production cost has not sufficiently been achieved.
Further, the field effect mobility of TFT used in the specification is calculated from the following equation or an equation equivalent thereto.μFE=(LdOX/WS∈VD)×(dID/dVG)where L designates a channel length of TFT, W designates a channel width, S designates a channel area, dOX designates a gate insulating film thickness, ∈ designates the dielectric constant of a gate insulating film, dVG designates a change in gate voltage, dID designates a change in drain current and VD designates drain voltage (which is set to 1 V). As is apparent from the equation, although the field effect mobility μFE is changed by being dependent on VG, the mobility of TFT indicates a maximum value of the mobility μFE (refer to FIG. 8A).
According to a semiconductor device with a digital signal as an input signal, for example, a liquid crystal display device of an active matrix type, an EL display device or the like, a method of integrally forming the device by using TFTs in a pixel matrix unit and a drive circuit is very valuable in view of reduction in production steps and expense. It is requested to sufficiently achieve the advantage.